The present invention relates, in general, to a method and apparatus for providing a wavelength selectable device, and more particularly to an integrated laser device having multiple lasers of different cavity lengths and/or different active layer bandgaps, selectively activated and coupled to an output to produce an output beam having a wavelength corresponding to the cavity length and/or to the active layer bandgap of the selected laser. The laser cavity is defined through the etching of its mirror facets.
Advances in current monolithic integration technology have allowed lasers of complicated geometry to be fabricated, including ring lasers with a variety of cavity configurations. These developments expand the prospective applications for integrated semiconductor lasers and add the attractiveness greater manufacturability and reduced cost. This technology has opened the opportunity to explore new and novel features that can be combined with integrated semiconductor laser devices, both inside and outside the laser cavity.
A ring cavity laser possesses benefits that a Fabry Perot cavity does not provide; for example, a ring cavity will produce lasing action with higher spectral purity than can be obtained with a Fabry Perot cavity. Such a ring cavity is illustrated by the monolithic triangular ring laser described in U.S. Pat. No. 4,851,368, the disclosure of which is hereby incorporated herein by reference. The coupling of ring lasers to form optical logic circuits has been described in U.S. Pat. No. 5,132,983 and in U.S. patent application Ser. No. 09/918,544 entitled xe2x80x9cCurved Waveguide Ring Lasers,xe2x80x9d the disclosures of which are hereby incorporated herein by reference. Unidirectional behavior in ring lasers has been described in U.S. Pat. No. 5,132,983, in U.S. Pat. No. 5,764,681 and also in U.S. patent application Ser. No. 09/918,548 entitled xe2x80x9cUnidirectional Curved Ring Lasersxe2x80x9d, the disclosures of which are hereby incorporated herein by reference.
Over the past few years, thanks mainly to the popularity of the Internet, the demand for bandwidth in optical communication systems has experienced explosive growth. Carrier companies and their suppliers have addressed this demand by installing Wavelength Division Multiplexing (WDM) systems that allow multiple wavelengths of light to be transmitted through a single strand of optical fiber. However, this, in turn, has given rise to a demand for many different lasers, each emitting light of a specific wavelength. As a result, the carrier companies have had to stock many lasers, each with a different output wavelength, and this has created a tremendous inventory problem for them.
A laser source capable of emitting multiple wavelengths in accordance with the present invention would address the foregoing problem, and, in addition, could be used in Wave Division Multiplexing (WDM) systems as a spare device that could operate at one of the multiple wavelengths. Furthermore, a wavelength selectable device that could be set remotely to emit one of its selectable wavelengths would allow a provider of service to light up an unused wavelength or reprovision a wavelength without needing a service call. This would lower provisioning costs tremendously for a service provider.
A wavelength selectable device would also be desirable in an application such as wavelength routing, in which the inexpensive generation of multiple wavelengths would allow widespread deployment of such devices in data communications. One example of using such routing of traffic is in Passive Optical Networks (PONs), where no active elements are present in the path of the signal between the source and the destination.
Briefly, the present invention is directed to an integrated semiconductor laser device that is capable of selectively emitting several wavelengths. The device includes multiple ring lasers of different cavity lengths selectively activated and coupled to an output to produce an output beam having a wavelength corresponding to the cavity length of the selected ring laser. Alternatively, or in conjunction with the foregoing device, ring lasers with different active layer bandgaps are selectively activated to emit different wavelengths.
In a first embodiment of the invention, different cavity length ring lasers and/or different active layer bandgap ring lasers are coupled in series, or in cascade, the ring lasers each preferably being unidirectional. The lasers are of different cavity lengths and/or of different active layer bandgaps so that each laser emits light at a different wavelength, and they are positioned so that the output light beam emitted from a first laser is coupled into the input of the next laser. The combined lasers produce a wavelength selectable device that is able to emit light of a wavelength corresponding to a selected one of the cascaded ring lasers. The active laser is chosen by biasing it above its threshold, while the remaining different length lasers are biased below threshold and the different active layer bandgap lasers remain unbiased. The ring lasers that are not biased above threshold are transparent to the light emitted by the selected ring laser so the light produced by the selected laser is propagated from the selected device, through any intervening lasers, to the device output.
In a second embodiment of the invention, a semiconductor optical amplifier (SOA) element is provided at the device output. The SOA element is switched between an above-zero bias and a reverse bias to modulate light emitted by any of the ring lasers and inserted into the SOA element. This SOA element absorbs the received light under reverse bias and allows the light to pass through without loss, or amplifies the light, under positive bias. The reason for operating the SOA element in this way is to provide a high-speed modulation capability at the wavelength selectable device output. The SOA receives the inserted light, optionally amplifies it, passes it unchanged, or absorbs it, and provides a modulated output at the wavelength of the activated laser. Alternatively, an electroabsorption (EA) element can be substituted instead of the SOA element and the modulation performed through the application of zero to reverse bias on the EA element. The utility of the integrated structure of the invention is in its ability to produce a laser light output beam of a predetermined wavelength at a predetermined location, and to modulate the output beam, if desired.
In a third embodiment of the invention, multiple ring lasers, preferably of the unidirectional kind, are each connected to a common, multi-legged output cavity either directly or through an intermediate SOA. In this way, multiple lasers can be simultaneously activated and, if desired, each laser""s output can be modulated before it enters the multi-legged output cavity, which may also be a SOA. This integrated device is used in applications where more than a single wavelength is desired, such as in Wavelength Division Multiplexing (WDM). Alternatively, an EA element can be substituted for the intermediate SOA element and the modulation performed through the application of zero to reverse bias on the EA element.
The multi-legged output cavity of the foregoing embodiment provides a single output beam; however, in another embodiment, the output cavity has multiple output points for greater fan-out applications. This embodiment can be used for applications desiring multiple output beams, and couples one or more wavelengths before the beam is emitted from the device.